1. Field of the Invention
The present invention relates to excimer laser devices, and particularly to an excimer laser device which excites laser gas in a chamber to produce a laser beam.
2. Description of the Background Art
FIG. 11 is a cross sectional view illustrating an entire structure of a conventional excimer laser device.
Referring to FIG. 11, a laser medium gas (hereinafter referred to as laser gas) containing halogen gas is enclosed in a laser chamber 81, and a predetermined high voltage is applied between discharge electrodes 82a and 82b to cause discharge and excite the laser gas so as to generate a laser beam. The laser beam thus generated is emitted to the outside of laser chamber 81 through windows 83 and 84 placed in the side walls in the direction of an optical axis. A fan 85 provided at a predetermined position in laser chamber 81 circulates the laser gas in laser chamber 81 to direct the laser gas to the location between discharge electrodes 82a and 82b. 
A motor 87 for driving fan 85 to rotate it about an shaft of rotation 86 is located on the outside of laser chamber 81. Shaft of rotation 86 passes through the right and left side walls of laser chamber 81 and is rotatably supported by bearings 88 and 89 respectively.
There are further provided on the outside of laser chamber 81, a gas-dust separation unit 90 and a gas passage 91 for establishing communication between the inside of laser chamber 81 and the inside of gas-dust separation unit 90. The laser gas circulated by fan 85 is passed into gas-dust separation unit 90 through gas passage 91 so as to remove dust from the gas. Then, the gas cleaned through gas-dust separation unit 90 is returned into laser chamber 81 through a gas passage 92 formed in the side walls of laser chamber 81. At this time, the cleaned laser gas is partially returned through a labyrinth portion 93 located inside and near windows 83 and 84 into laser chamber 81, and the remaining gas is returned to fan 85 along gas passages 94 and 95, bearings 88 and 89 and shaft of rotation 86.
The conventional excimer laser device employs ball bearings as bearings 88 and 89 for fan 85. This device is accordingly disadvantageous in that impurity gas generated by reaction between the halogen gas in the laser gas and the lubricant for bearings, or the dust generated from the ball travelling surfaces of bearings 88 and 89 could deteriorate the laser gas and the resultant laser output could decrease. Further, bearings 88 and 89 themselves could wear due to the operation over a long period of time, requiring a regular maintenance.
In order to address this deficiency, the inventor of the present application has proposed a device which provides a non-contact support of rotational shaft 86 of fan 85 by magnetic bearings. This device allows rotational shaft 86 of fan 85 to be rotationally driven with no contact, so that the impurity gas and dust are never generated.
However, if any magnetic material is included in the dust generated during laser discharge or the like, the dust could adhere to and accumulate on a magnetic pole portion of an electromagnet of the magnetic bearing and accordingly, control of the magnetic bearing could be affected adversely.
A further problem of this device is increase in the size thereof due to the arrangement of rotational shaft 86 and magnetic bearings on both sides of fan 85.
One object of the present invention is to provide an excimer laser device capable of preventing accumulation of dust on a magnetic bearing of a laser gas circulation fan.
Another object of the invention is to provide an excimer laser device of a small size in which laser gas is never contaminated.
According to one aspect of the invention, a magnetic field erasing unit is provided for erasing a residual magnetic field by applying an alternating current to an electromagnet of the magnetic bearing when the excimer laser device is stopped. In this way, it is possible to weaken the adhesiveness of dust to the electromagnet and accordingly prevent the dust from accumulating on the electromagnet.
Preferably, a dust sucking unit is further provided for sucking and thus removing the dust adhering to the electromagnet of the magnetic bearing. In this way, the dust adhering to the electromagnet can easily be removed.
According to another aspect of the invention, the laser gas circulation fan is formed of a cylindrical member and vane members placed around the circumference of the cylindrical member, with a fixed shaft inserted into the cylindrical member. The fan is supported from the inside of the circumference of the cylindrical member with no contact by the magnetic bearing provided to the fixed shaft. The fan is driven to rotate by a driving unit with no contact. Accordingly, no impurity gas and no dust are generated from the bearing as they occur in the conventional device employing ball bearings or the like as the bearing, and the laser gas is never contaminated. Further, the device size can be made smaller compared with the conventional device having its shaft of rotation projecting from both sides of the fan.
Preferably, the magnetic bearing includes a radial displacement sensor for detecting the radial position of the fan and a radial electromagnet with its coil current controlled according to the result of detection by the radial displacement sensor. The magnetic bearing is thus constructed easily.
More preferably, the driving unit includes a rotor shaped into a ring along the inside of the circumference of the cylindrical member of the fan and a stator fixed onto the fixed shaft opposite to the rotor so as to generate a rotating magnetic field and thus drive the rotor to rotate with no contact. The driving unit is accordingly constructed easily.
Still preferably, a cable hole is made from the outer wall of the chamber toward the inside of the shaft of rotation, and a cable for the magnetic bearing and the stator is drown through the cable hole to the outside of the chamber. In this way, damage to the cable by the laser gas as well as contamination of the laser gas caused by impurity gas generated from the cable can be prevented.
Still more preferably, a metal cover is further provided for protecting the magnetic bearing and the stator. It is thus possible to protect the magnetic bearing from laser gas and noise and prevent generation of impurity gas from the stator.